1. Field of the Invention
The present invention relates to a tracking servo control system for an information read-out system, and more specifically to a system capable of a quick and certain lock-in of the tracking servo operation.
2. Description of Background Information
In a system for reading-out recorded information, such as a video disc player system and PCM disc or compact disc player system, an information detection point, i.e., a point at which reading-out of the information is taking place, is likely to deviate from the recording track, in a radial direction of the recording disc. This is due to the presence of an eccentricity, that is, the discrepancy between the center of the disc rotation at the time of recording and the center of the disc rotation during playback operation. This eccentricity can also be caused by a precession, that is, the motion of the center of the disc, while the disc is rotating in the information read-out system.
The tracking servo system is utilized in the information read-out system in order to apparently cancel this discrepancy and to make the detection point to trace the center of the recording track correctly.
The tracking servo system generally includes means for generating a tracking error signal having a level and a polality representing the magnitude and the direction of the displacement of the information detection point from the center of the recording track. Then, the information detection point is forcedly moved in the radial direction of the disc in accordance with this tracking servo signal.
Further, in order to realize a stable operation of the system, the tracking servo system is provided with an open loop operation which is utilized during an initiation period of the tracking servo operation. However, a problem of the conventional tracking servo systems is that there is a difference in the phase of the tracking error signal and the phase of a control signal for commanding the switch over between the open loop operation and the closed loop operation of the tracking servo system. Another problem of the conventional tracking servo systems is that the speed of the lead-in operation of those systems is relatively slow.